1. Field of the Invention
The present invention is related to controls for gas turbines and, more particularly, to an improved control system for a gas turbine power plant which can be adapted for use on either single-shaft or two-shaft machines.
2. Description of the Prior Art
Controlling the various parameters in a large gas turbine has become quite complex, but, as a result of the increasing complexity, the output characteristics have been enhanced together with an increased lifetime for many of the component parts. Such controls normally incorporate means to influence the rate of fuel flow to the gas turbine combustion chamber in accordance with fuel control signals obtained by monitoring the operating parameters of the gas turbine. Examples of such control systems are known in the art and may be found in U.S. Pat. No. 3,520,133 issued July 14, 1970 to A. L. Loft et al for a "Gas Turbine Control System;" U.S. Pat. No. 3,639,076 issued Feb. 1, 1972 to W. I. Rowen for a "Constant Power Control System for a Gas Turbine" and U.S. Pat. No. 3,729,928 issued May 1, 1973 to W. Rowen for a "Torque Control System for a Gas Turbine," all assigned to the assignee of the present invention, and all of which are expressly incorporated herein by reference.
In a single-shaft gas turbine, the method normally utilized to control the output of the machine is to control the amount of fuel delivered to the combustion chamber, which results in changes in the firing temperature. If the firing temperature is changed very rapidly, which can occur in cyclic load applications, thermal stresses are created in the hot gas path parts, such as the turbine blades, since they are not heated evenly in response to rapid changes in gas temperature. Excessive application of thermal stresses can lead to thermal shock which is a condition wherein the thermal stresses that are generated exceed the elastic mechanical strength of the material. Thermal shock, in turn, can lead to component failure requiring expensive shut-down and maintenance.
Thermal shock can, of course, be avoided by minimizing severe or cyclic temperature changes by, for example, changing the load very slowly. However, there are many applications where the input of the gas turbine cannot be controlled, but must respond to external influences. One example is a single-shaft gas turbine connected to an isolated generator powering an arc furnace. Another example is a single-shaft gas turbine connected to an isolated generator for driving a power shovel in a mine. Yet another example is a two-shaft gas turbine for providing propulsion for an ice-breaking marine vessel. In the latter instance, full power is desired to drive the vessel through the ice until it can go no farther; the propeller is then stopped (the turbine is unloaded) and reversed to remove the ship from the ice to get another running start. It can be appreciated that such a mode of operation results in a cyclic load application to the gas turbine that can result in the above-described thermal fatigue.
It is toward overcoming the problems associated with thermal stresses resulting from cyclic load applications to both single and two-shaft gas turbines that the present invention is advanced.
Known gas turbines employ several different mechanisms for controlling air flow. For example, variable inlet guide vanes can be positioned at the inlet of the compressor for air flow control during start up of the gas turbine to prevent pulsation or surge in the compressor. The guide vanes are utilized to throttle the air to prevent such instabilities until the gas turbine is at full speed. Inlet guide vanes are also utilized to allow maintenance of high exhaust temperatures at part load for heat recovery purposes.
Blow off valves can be positioned in known gas turbines between one stage of the compressor and the gas turbine exhaust and are presently utilized to limit the amount of overspeed in applications where load can be lost instantly.
Another type of air control present in two-shaft gas turbines comprises a variable area turbine nozzle which controls division of energy between the high pressure compressor turbine (and therefore compressor speed) and the low pressure or load turbine for the purpose of optimizing heat rate. The control of the variable area turbine nozzle affects air flow only because it permits variable speed control of the high pressure compressor set.
In known gas turbines utilizing variable inlet guide vanes, blow off valves or variable area nozzles for controlling air flow in and around the compressor, the primary means of controlling turbine output is by varying fuel flow, as described in the above-cited United States Patents.